Throat microphones, which convert vibrations of throats into audio signals, have an advantage in that they can precisely convert human voices into audio signals even in noisy environments. A typical throat microphone includes an acoustoelectric transducer consisting of a piezoelectric device. Such a transducer of the throat microphone generally includes a compact piezoelectric bimorph that can output high levels of signals in response to displacement.
FIGS. 3 and 4 illustrate a technology on a throat microphone including a piezoelectric bimorph. In the drawings, a piezoelectric device 50 consisting of a piezoelectric bimorph has a fixed end 51. The fixed end 51 is embedded in a stationary part 62 that protrudes from a base 63. The piezoelectric device 50 is cantilevered in such a manner that the piezoelectric device 50 extends parallel to a surface of the base 63 and can vibrate perpendicular to the surface of the base 63.
The piezoelectric device 50 consisting of a piezoelectric bimorph produces electrical signals in proportion to vibrational displacements. The piezoelectric device 50, which outputs signals proportional to the displacements, is called “displacement-proportional device”. The piezoelectric device 50 is deformed by external force and resiliently returns to its original shape when the force disappears. In other words, the piezoelectric device 50 is deformed by acceleration input and output signals. Such a piezoelectric device 50 is called “stiffness control device”. As shown in FIGS. 3 and 4, the piezoelectric device 50 has a free end 52 provided with an anchor 64 fixed thereto, so that the piezoelectric device 50 can efficiently respond to vibrational acceleration.
A throat microphone is designed to have a resonant frequency equal to the upper limit of the frequency band of the collected sound. Such a design equalizes the frequency response of the output signals corresponding to acceleration applied to the piezoelectric device 50 in the throat microphone. Since the throat microphone detects the vibrational acceleration from the throat with a piezoelectric element, the piezoelectric device 50 has a resonant frequency of 3-4 kHz which is equal to the upper limit of the frequency band of the collected sound. The resonant frequency of the piezoelectric device 50 depends on the stiffness of the piezoelectric device 50 and the mass of the anchor 64. For the piezoelectric device 50 (which is of a stiffness control type) with a constant stiffness, the resonant frequency decreases and the sensitivity to the acceleration increases in proportion to the mass of the anchor 64. Such relation between the anchor 64 and the resonant frequency and sensitivity is disclosed in Japanese Unexamined Patent Application Publication No.2012-231204.
Since the piezoelectric device 50 is designed to have a resonant frequency of 3-4 kHz as described above, the throat microphone produces audio signals with high clarity at high sensitivity to the vibrational acceleration from the throat within the audio frequency band. The frequency response at the resonant frequency depends greatly on the sharpness of the resonance (Q).
The device of FIGS. 3 and 4 includes a low-resilience silicone viscoelastic rubber piece 65 between the base 63 and the piezoelectric device 50. The viscoelastic rubber piece 65 attenuates the vibrations of the vibration system. As such, the traditional structure (FIGS. 3 and 4) is provided with the viscoelastic rubber piece 65 to decrease the sharpness of resonance (Q) and sensitivity (at the resonant frequency) of the piezoelectric device 50, aiming at collecting high-quality audio signals even in noisy environments. The structures in Japanese Examined Utility Model Application Publication No. S63-49018 and Japanese examined Patent Application H4-32599 each include a hermetically-sealed container loaded with a piezoelectric device and a viscous liquid damper. These structures are aimed at decreasing the sharpness of the resonance of the piezoelectric device 50, like the structure in FIGS. 3 and 4.
The viscoelastic rubber piece 65 of the device of FIGS. 3 and 4 is prone to variations in contact, i.e., mechanical coupling to the surfaces of the base 63 and the piezoelectric device 50. Such variations lead to a difference in vibration propagation to the piezoelectric device 50 among finished products in the structure shown in FIGS. 3 and 4. To address such a situation, the vibration system in FIGS. 3 and 4 is entirely covered with a sealant 66 of room-temperature vulcanizing (RTV) rubber to improve the mechanical coupling between the base 63 and the piezoelectric device 50. Unfortunately, the sealant 66 covering the overall vibration system in FIGS. 3 and 4 cannot readily reduce a difference in vibration propagation to the piezoelectric device 50 among the finished products.
In FIGS. 3 and 4, the piezoelectric device 50 outputs audio signals through lead connections 67 and leads 68.
Japanese Unexamined Patent Application Publication No. H10-79999 evidentially shows that a piezoelectric device in a conventional throat microphone vibrates perpendicular to a surface of the base.